RESIDENT EXPERT Our series on
artificial intelligence considers the 1307
development of expert systems — programs
that incorporate highly specialised areas of
knowledge
COMEBACK BID Weighing in with a
stylish design, large memory and seductive 1309
price, the Atari 130XE looks like it could
have the makings of a champ
EXTENDED VIEW We take a look at how
CP/M labels and manipulates its files _ 1304
ICON TACT Beyond Software’s
Shadowfire uses Mac-like icons as part of a 1370
poems fast-action game : 1) The Atari 130XE possesses one more chip than its
= _____ predecessors. What is its name and function?
2) What is the general effect of a CALL command in
780 Assembly language?
3) Whatis a ‘wildcard’?
COMPUTER SCIENCE
SEARCHING LOOKS A sisctanins ne
PROLOG’S unique tree-like search procedure
~ FROM PET TO PLOTTER A weekly
glossary of computing terms 1308
THE NEW WORLD I The first in a three-
part complete listing of our New World 137 4 |
simulation game. This week we provide
flavours for the Spectrum
BM PC COURTESY OF CUP ERE 114 CHARING CROSS ROAD, LONDON WC2
CHANNELS OF THOUGHT We
investigate how the Spectrum sends data to 137 A
the screen and ZX printer via channels
DIGITAL DOODLES Our digital tracer
project concludes by adding some 1306
refinements to the control program we gave
last week
REFERENCE CARD Another section of - INSIDE
the Commodore 64’s memory map | us
COVER PHOTOGRAPHY BY MARCUS WILSON SMITH
RESIDENT EXPERT
This instalment of our series on artificial
intelligence focuses on expert systems —
highly structured programs capable of
fulfillimg much of the diagnostic and
advisory role of professional human
practitioners in specialised areas of
knowledge. We _ isolate the _ essential
ingredients of the true expert system.
A human expert — a hospital consultant, a
geologist, or a chemical analyst, for example — is
usually someone we respect. Experts spend a long
time studying and practising their skills to enable
them to do their jobs well. The trouble with human
experts, however, is that they are scarce, they are
not always reliable, they want payment, and in the,
long run they die, taking much of their expertise
with them. Many people are therefore keen on the
idea of encoding expertise in computer programs,
to get the benefits of being able to draw on a large
body of knowledge without the drawbacks
associated with the human experts.
The concept of the ‘expert system’ arose in the
1970s, when artificial intelligence (AI) researchers
abandoned, or postponed, the quest for generally
intelligent machines and turned instead to the
solution of narrowly focused real-world problems.
Thus the expert system is one of the first examples
of applied AI, and expert systems techniques have
spread out far beyond the confines of the research
laboratories in which they were devised. Indeed,
expert systems have to an extent brought AI into
practical everyday use. Systems already exist that
out-perform skilled humans at medical diagnosis,
mass-spectrogram interpretation, classifying crop
disease and much else besides. It is worth asking,
therefore, how they work.
WHAT IS AN EXPERT SYSTEM?
Typically an expert system is based on an extensive
body of knowledge about a specific problem area.
In general, this knowledge will be organised as a
collection of rules that allow the system to draw
conclusions from given data or premises, thereby
enabling it to offer intelligent advice or take
intelligent decisions. This knowledge-based
approach to systems design represents an
evolutionary change within computer science,
with revolutionary consequences. It replaces the
traditional formula of ‘Data + Algorithm =
Program’ with a new architecture centred around a
‘knowledge base’ and an ‘inference engine’, so that
‘Knowledge + Inference = Expert System’. This
formula is obviously similar, but different enough
in approach to have profound implications.
MARCUS WILSON-SMITH
What is an expert system? The following
checklist of typical features should prove helpful.
@An expert system is limited to a relatively narrow
domain of expertise.
@It should be able to reason with uncertain data
and unreliable rules.
et must be able to explain its train of reasoning in ©
a comprehensive way.
@F acts and inference mechanism are ‘detachable’:
knowledge is not ‘hard-coded’ into the deductive
procedures.
elt is designed to grow incrementally.
@It is typically rule-based.
@It delivers advice as its output — not tables of
figures or graphs.
The key word is knowledge. Clearly the objective
of an intelligent problem-solving system is to cut
out blind or random search. To do so a computer
system has to exploit the same advantage that the
human expert has over the novice — that is,
expertise, or organised knowledge: knowledge
about facts, about rules of inference and about
solution strategies. There are four essential
components of a fully fledged expert system:
1. The knowledge base
2. The inference engine
3. The knowledge-acquisition module
4. The explanatory interface
Seeking Professional Advice
Systems that can acquire
knowledge from experts and use
that knowledge to proffer advice
or diagnoses are becoming
increasingly popular in many
disciplines —.from medicine to
farming and architecture. Expert
systems, designed to operate
within a narrow subject area,
provide professionals with an
on-line consultancy service to
assist them in their work
THE HOME COMPUTER ADVANCED COURSE 1301
HSOLNIOVW JHL NO TIANNINSW NVI
System Synopsis
_ Anexpert system comprises
several modules that allow
knowledge to pass from the
expert to the end user.
Knowledge must first be
acquired from the expert(s) and
incorporated into a knowledge -
base. In order to make
predictions, give advice or
provide a diagnosis, the system
must then be able to draw
inferences from the knowledge
base. Finally, the explanatory
_ interface allows the user to
communicate with the system in
order to consult it
All four modules are critical. A knowledge-based
system may lack one or other of them, but a truly
expert system should not. We will consider each of
these four modules in turn and explain how they
work together.
THE KNOWLEDGE BASE
The two fundamental components of an expert
system are the knowledge base and the inference
engine. The knowledge base stores information
about the subject domain; however, information
in a knowledge base is not the passive set of
records and items that you would find in a
conventional database. Instead it contains
symbolic representations of experts’ rules of
judgment and experience in a form that enables
the inference engine to perform logical deductions
from it.
Most of the items in a knowledge base are non-
mathematical. The two chief difficulties in
developing a knowledge base are knowledge
representation and knowledge acquisition. ‘The
former problem concerns the decision on how to
encode knowledge so that the computer can use it.
In general the following elements must be
represented: domain terms (the jargon used by
experts in the field); structural relationships (the
interconnections of component entities); and
1302 THE HOME COMPUTER ADVANCED COURSE
causal relationships (the cause-effect relations
between components).
The task of the knowledge engineer is to select
appropriate means of storing such information
symbolically. Four main methods have evolved:
@®Rules in IF...THEN format. The condition
specifies some pattern and the conclusion may be
an action or assertion. |
@ Semantic nets. These represent relations among
objects in the domain (e.g. the whale is amammal)
by links between nodes.
®Frames. These are generalised record structures
which may have default values and may have
actions coded as the values of certain fields or
slots.
® Horn clauses. This is a form of predicate logic on
which PROLOG is based and with which the PROLOG
system can perform inferences (see page 1272).
Early expert systems used the rule-based
formalism almost exclusively. A sample rule from
the Mycin system for diagnosing blood infections
is typical of the IF. . .THEN structure:
IF:
1. The infection requiring therapy is meningitis, and
2. The type of infection is fungal, and
3. Organisms were not seen on the stain of the
culture, and
4. The patient is not a compromised host, and
5. The patient has been in a region where
coccidiomycoses are endemic, and
6. The race of the patient is black or Asian or Indian,
and
7. The cryptococcal antigen in the csf was not positive
THEN
There is suggestive evidence that cryptococcus is not
one of the organisms which might be causing the
infection.
From this example we can see that an expert
system uses the technical jargon of the area in
which it is designed to operate — in this case
medicine. The IF. . .THEN construct used by Mycin
is essentially a series of statements that can be
determined as true or false. Thus, the statements
can be linked by Boolean operators, such as AND,
to assist computer manipulation. In order to elicit
the information required to make a diagnosis,
Mycin must enter into a dialogue with the system
user. Obviously, in this system at least, the user
must have a certain level of knowledge in the
subject area, so that the expert system’s queries
can be understood and answered.
THE INFERENCE ENGINE
The inference mechanisms consist of search and
reasoning methods that enable the system to find
solutions and, if necessary, provide justifications
for its answers. There are two overall reasoning
strategies — forward chaining and backward
chaining. !
Forward chaining involves working forwards
from the evidence (or symptoms) to the
conclusions (or diagnoses). In a rule-based system
it simply involves matching the IF conditions to the
facts, possibly in a predetermined order. Forward
chaining is easy to computerise and is suitable in
cases where all the data is to be gathered anyway.
Examples of such cases are where the data is
generated automatically by an instrument and
where a form has to be filled in.
_ Backward chaining works from hypothesis to
evidence. The system chooses a hypothesis and
looks for data to support or refute it. It can be
programmed in a recursive manner and in
consultation-style systems typically leads to a
more natural kind of dialogue. The problem of
which hypothesis to pick in any given situation is
not yet fully solved and so in practice most systems
use a mixture of forward and backward chaining.
THE ACQUISITION MODULE
Experts are notoriously bad at saying how they
reach their conclusions, not necessarily because
they wish to preserve trade secrets but because
many of their thought processes lie buried beneath
the level of consciousness, at the intuitive level. So
knowledge acquisition has come to be regarded as
the main bottleneck in expert systems
development. Experts tend to be good at criticism,
however. They can look at an example case and
say what decision should have been taken and, if
required, criticise a computer’s suggested solution.
A good deal of attention has therefore been
devoted recently to developing software tools that
allow an expert system to induce its own
knowledge from _ pre-classified examples.
Effectively, this power assists the knowledge
acquisition process. Overcoming at the same time
many of the difficulties of extracting knowledge
from human experts and the laborious task of
coding it for the computer. Even if the system
cannot do the whole job — from a database of
instances to a set of decision rules — on its own, it
may be useful if it can refine its own knowledge
base during a period of ‘apprenticeship’ or during
use, learning from its mistakes. We will take a
closer look at some machine learning systems in
forthcoming instalments.
THE EXPLANATORY INTERFACE
One of the good things about Mycin, Prospector
and many classic expert systems was that they
could justify their conclusions on _ request.
Obviously when the responsibility for life-and-
death or even profit-and-loss decisions is being
handed over to the computer it is essential for the
system to explain its reasoning. Here is an example
of how Mycin replies to a WHY? question posed by
its user (who is a physician, not a patient, and will
not accept diagnoses on trust without
explanation). The dialogue starts by Mycin
requesting data.
Is meningitis a hospital-acquired infection?
WHY?
[2.0] This will aid in determining the organisms (other
than those seen on cultures or smears) which might
t&
8
Bi L _
be causing the infection. It has already been
determined that:
[21] the infection which requires therapy is
meningitis, and
[2.2] organisms were not seen on the stain of this
pending csf culture, and
[2.3] the type of infection is bacterial, and
[2.4] neurosurgery is not one of the relevant items
from the history of this patient, and
[2.5] the age of this patient is greater than 10 days.
Therefore, if:
[2.6] the infection was acquired while the patient was
hospitalised,
then there is evidence that the organisms (other than
those seen on cultures and smears) which might be
causing the infection are e.coli (0.75),
Staphylococcus-coag-pos (0.3), pseudomonas-
aeruginosa (0.3), klebsiella-pneumoniae (0.5)
— from [RULE 545]
This explanation facility is in effect a partial trace
through the program’s reasoning process, couched
in medical-jargon English. Such explanations can
be produced fairly easily and cheaply in rule-
based systems (by regurgitating the rules being
used and their predecessors), which is a point in
favour of the rule-based encoding of knowledge.
Notice that the conclusions have numerical
weightings attached. These are in fact not true
probabilities. They are weightings that allow the
system to deal with uncertainty in a consistent
manner, and come up with an ordered list of likely
diagnoses in the final analysis.
mand he.
© Special
ys
0:
Hs
; |
_
te
.
Hy
COURTESY OF ICI PLANT PROTECTION UK DIVISION
SSeS
SSeS
The Grain Brain
Wheat Counsellor is an expert
system developed by ICI and
available to farmers, free of
charge, on Prestel. The system
is designed to advise farmers
on methods to combat wheat
diseases, the correct chemicals
to apply and an estimate of the
likely losses. Although the
system is designed to simulate
the type of conversation that
might be held over a farm gate,
its knowledge has been
accumulated from scientific
studies of the spread of crop
diseases. In order to gain the
information from which it will
draw its conclusions, the
system leads the farmer through
a series of simple questions. Its
advice is then given in the form
of a list of the treatments
available, together with a
suitability rating for each
treatment:
THE HOME COMPUTER ADVANCED COURSE 1303
EXTENDED VIEW
So far in in 1 our series Dr n CP/M w we e have concentrated
primarily on looking at the command structure of the
operating system. We turn our attention now to the
files upon which CP/M commands operate and how
they can he manipulated within the system.
We have already discussed file extensions in CP/M, but
- let’snowlookatthem in greater detail. As we have seen, a
_ CP/M file consists of a primary name, which can contain
up to eight characters, followed by a full stop and up to
three letters, constituting a file extension (although this
may be omitted). You can theoretically add any extension
onto the primary filename, butsome of these are reserved
for specific purposes. For example, the COM extension is
reserved for CP/M COMmand files. This informs CP/M
that a file with this extension is to be added to the list. of
transient commands that can be run under it.
Similarly, Basic programs written under CP/M qin
be stored with the file extension .BAS. Many versions of
pasic that run under CP/M will automatically assign this
~ extension to a sasic file, eliminating any need for you to
type it in. Although the Basic program will be stored as a
source (text) file, the difference between this file and an
ordinary text file is that when the program is reloaded
prior to being run, CP/M will tokenise the Basic program
rather than list it as an ASCII sequential file. However,
many machines will compile their sasic programs before
running them. Files that are compiled before execution
will be stored with an .INT extension. This means that
they are INTermediary files consisting of object code.
Machine code programs, like Basic programs, can also
have several different extensions assigned to them
depending on their status. When we write a program in
Assembly language, the source listing should have the
extension .ASM attached. If the program does not have
the correct extension, the resident assembler will not
1304 THE HOME COMPUTER ADVANCED COURSE
attempt to assemble the program am will simply
generate a file error.
When we attempt to assemble the program, we must
select one of two extensions. These are either .HEX or
PRN. The HEX extension means that what is produced at
the end of assembly will be a HEXadecimal object file.
Alternatively, with PRN the assembler will also produce a
printed listing of the assembly with a copy of the source
listing, the object code, the addresses of each of the op-
codes and the list of assignments along with a list of
errors. Thus, the PRN extension provides an essential aid
in debugging assembly programs.
The final group of file extensions we shall look at are
those associated with text files. Many versions of CP/M
have a small text editing facility (called by executing the
command file ED) provided within the operating system
itself, whereas others assume you will have a specialised
word processing package that can be run under CP/M.
However, regardless of which you may be using, the file
extensions remain the same.
Apart from the reserved filename extensions that have
already been mentioned, almost any three letters can be
used as an extension. In fact, you do not have to add an
extension at all. If a text file is SAVEd without an extension
having been added, CP/M will often add its own. For
example, the TEXTED utility will add the extension .TXT
on the end of a file whereas WordStar will add .SSS.
BACKUP FILES
lf you look atthe directory when atextfile has been SAVEd
you will notice that two files have been created: one with
the file extension you have added and one with the suffix
.BAK. This is a built-in safety feature of CP/M. Obviously,
it is vital that important documents are not accidentally
deleted or corrupted, so to prevent such disastrous
occurrences CP/M will create two copies of all text files: a
normal file and a backup .BAK file.
Thus, if you accidentally ERAse a file when editing, the
backup version will always be available so that your
previous work will not be irretrievably lost. When a file
has been edited and then reSAVEd, a new version of .BAK
will also be created. Itis therefore a good idea to regularly
reSAVE files so that the backup copy will be kept up to
date.
Despite the fact that it is not necessary to add
extensions, once implemented they are a useful aid in
organising your files; not only for your own benefit — for
displaying information about the organisation of files and
headings — but also for the manipulation of groups of
files. One of the most useful of these manipulations
involves the use of ‘wildcard’ characters, which enable
‘fuzzy matching’ of files.
Suppose, for example, that a managing director has
written a number of memoranda in the month of July.
Sometime later he may decide these files are out of date
and no longer needed in his catalogue. To delete July's
memoranda, he could, of course, go through the
catalogue and delete each of the files individually using
the ERA command, but if there are a large number of files
this could be a long and tedious process.
However, providing he has had the foresight to give all
July’s memos the same file extension, say .JUL, this
process could be performed with a single command: ERA
D:*.JUL (where D is the optional drive name). Placing the
asterisk before the full stop tells CP/M to ignore the
primary filename and ERAse the files with the JUL
extension. This asterisk can also be used on the right of
the full stop. Using asterisks on either side of the full stop,
the command will erase all the files on a disk, such as
ERAS.
SELECTIVE FILE DELETION
Let’s assume that the managing director keeps all July's
files with the suffix JUL. We shall now complicate
matters by also assuming that there are a number of
important documents that he does not want to delete. In
this case it is out of the question to use the * prefix, since
this would erase July’s important files along with the
redundant memos. However, our managing director has
distinguished his memoranda from other files by naming
them MEM1.JUL, MEM2.JUL and so on. However, it is
still possible to delete all the memoranda with a single
command without accidentally deleting other files, in
which case executing the command ERA D:MEM?.JUL
would be appropriate. Here, the operating system is
being told to delete all files beginning with MEM and
ending with the suffix .JUL. The ? in the fourth position
signifies to CP/M that the fourth character is not
significant and may be ignored.
This process can be adapted for any of the 11 possible
positions in afile name. Itis particularly useful should our
managing director wish to delete memos for both JUNe
and JULY, since both sets could be replaced by the format
MEM?.JU?. Combinations of * and ? can appear ina file
name so that ERA D:???7Q???.* would erase all files
with a Q in the fifth position of the prefix.
Use of the wildcard characters * and ? is not confined
to the ERAse command but can also be used with, among
others, the PIP, STAT and REN commands. Hence, in
order to transfer a number of files from one disk to
another, a typical command would be PIP B:=A*.”. This
command will find all the files on drive A that match the
format (which in this case would be all of them) and copy
them onto the disk in drive B. In this way we can use
wildcards to copy the entire contents of one disk to
another.
Thus far in the series we have looked at commands
that are used to control CP/M. However, the operating
system also makes extensive use of control characters to
perform many of its operations. Perhaps the most useful
and common control character is the CTRL-C
combination. Pressing these characters will produce a
‘warm boot’ to the system and reload CP/M into the
computer. This is important, not just when reloading CP/
M after a system crash, but also when inserting a new
disk into one of the drives.
Remember that CP/M keeps a copy of a disk’s ‘log’ in
the computer's memory. When we exchange disks in a
drive the operating system must be informed of the
change, otherwise it may generate errors. When wewarm
boot CP/M, it will re-log the contents of each disk drive,
which will enable us to continue. CTRL-C can also be
used to halt the execution of many progams, since CP/M
will automatically interrupt the system while it resets
itself.
Many of the other control characters are concerned
with editing text and have been adopted by a number of
word processors that run under CP/M. For example,
CTRL-H will delete the last character, whereas CTRL-U
and CTRL-X will delete an entire line. If a printer is
connected, text can be sent to it by typing CTRL-P.
Pressing CTRL-P a second time halts this operation.
Other control characters are used in CP/M, many of
which have been rendered obsolete by advances in
hardware and software. There are several reasons why
these characters have been retained even though their
functions appear to have been duplicated by other keys;
for example, most home computers now have a Delete
key fitted. Part of the explanation is historical. Many early
microcomputers did not have a Delete key or cursor keys,
so that screen editing facilities had to be incorporated
within the operating system.
Another reason for retaining the system is that of
compatibility. While the CTRL and alphabetic characters
have retained their ASCII equivalents over the years, the
newer keys may not have, and so the control characters
have remained to retain compatibility. Finally, a user who
has spent several years with CP/M will have grown
accustomed to using these keys. Altering the system now
would mean that he would haveto relearn control from the
beginning.
Having finished our brief overview of the commands
and files as implemented on CP/M, we can now carry out
most of the everyday functions used within the operating
system. In the next instalment, we'll delve deeper into
CP/M to see how it is constructed and how the program
interacts with the computer and its peripherals.
LIZ DIXON ON THE MACINTOSH
Walk On The Wild Side .
The use of ‘wildcard’ characters
is an important feature of CP/M.
By entering ‘wildcards’ at
different positions, the
computer will effectively ignore
any characters in that position
when searching the directory.
This means that by careful
allocation of filenames the user
can manipulate whole lists of
files which are classified under
different headings
THE HOME COMPUTER ADVANCED COURSE 1305
We complete our digital tracer project b
giving the second half of the tracer program
for the BBC Micro. Adding this to the listing
given in the previous instalment provides the
user with an alternative method of enabling
the tracer to produce points, lines, circles
and filled triangles.
The second part of the tracer program controls the
use of the tracer in an ‘elastic mode’. Rather than
simply fulfilling a freehand drawing function, the
tracer can be used, when elastic mode is selected,
to specify the start and end points of a line, the
centre and radius of a circle, or the three corners of
a filled triangle. Selection of each of these
functions, together with a point-plotting fuction, is
made by pressing the appropriate keys from the
menu displayed at the top of the screen. Save,
Load and Clear screen functions, similar to those
for the freehand mode outlined in the previous
- instalment, can also be selected from this menu.
The new section of program given here, therefore,
allows complex patterns to be created simply on
the screen, using the tracer and a few simple key-
presses. These patterns can be saved to disk or
tape to be reloaded later.
THE FOUR PROCEDURES
@ PROCpoint
Calling PROCdraw (a procedure given in the first
half of the program) with togflag set to one
provides us with a simple way of moving the cursor
around the screen without drawing lines. Setting
togflag to one ensures that pen-up mode is selected
before the procedure is called. This call is placed
within a loop that also looks for a keypress that
might terminate the loop or be used to change the
foreground colour. In addition, if the tracer button
is pressed then a point is plotted in the current
foreground colour at the current cursor position.
In order to stop double presses, the procedure
cannot continue after a button press has been
detected until the button is released.
@ PROCline
The first action of the procedure is to store the
current cursor co-ordinates, as these will be used
to locate the fixed end of the elastic line. The
cursor is then erased and a line drawn to a point
corresponding to a new cursor position. This is
drawn in Exclusive-OR plotting mode, using
GCOL3. Redrawing this line again later will erase it
and restore any background ‘data to its original
condition. Placing these two drawing actions
within a loop allows the user to try out various line
1306 THE HOME COMPUTER ADVANCED COURSE
q
:
’
}
/
:
positions, as though the line were an elastic band
attached at one end to a fixed point and at the
other to the tracer’s screen cursor. When satisfied
with a line’s position the user can ‘fix’ the line by
pressing the tracer button or M on the keyboard.
In this case the procedure omits to draw the line a
second time, leaving it unerased, before returning
to the main menu.
@ PROCTill
The fill procedure takes advantage of the BBC’s
triangle-filling feature, accessed by using a PLOT
85,x,y command. This command takes the last two
points visited and the point specified in the
command as the three corners of a triangle and fills
the shape with a solid block of colour. The
procedure uses a similar method to that used in
PROCpoint to allow the user to plot the three
corners of the triangle to be filled, using the tracer
button. The last two lines of the procedure revisit
the three points in order to fill the triangle shape.
@ PROCcircle
The final facility offered by the program allows the
user to specify the radius of a circle using the same
elastic band method as that used in PROCline. In
fact, PROCline is called from within the circle-
drawing procedure for exactly this purpose. Once
the radius is selected control returns to PROCcircle,
which must first erase the line left by PROCline and
erase the cursor before drawing the circle. The
circle’s centre will be held in x1 and x2, the cursor
co-ordinates on entry to the procedure, and
PROCline provides the co-ordinates of a point on
the circumference, (x2,y2). Using these two points,
the circle’s radius can be calculated and a standard
circle-drawing algorithm used to create the circle.
Digital Tracer Program Part 2
2350
2360
Zar
2380
press
2370
2400
‘2410
2430
2440
2450
2440
2470
2480
2490
2500
The following listing forms the
second part of the digital tracer
program for the BBC Micro. It
should be added to the first part
of the listing given on page
1288
DEF PROCelastic
PROCelastic_inform
REPEAT
anst=INKEYSC1):1F ans#<¢>"" THEN PROCelastic_
togflag=1:PROCdr aw
UNTIL exitflag=1
ENDPROC
DEF PROCelastic_inform
PROCcalc_xy:0)ldx=x:oldy=y:PROCcursor(oldx,aldy?
PROCreinform
GCOL 0,1:MOVE 0,920:DRAW 1280,%20
ENDPROC
DEF PROCreinform
PRINT TABC1,1);SPCC79)
PRINT TABC1,1293;"S=Save L=Load M=Menu C=Clear
R=Circle"
2510
PRINT" D=Draw Line
P=Point F=Fili"
ENDPROC
DEF PROCelastic_press
IF ans#="C" THEN CLS:PROCelastic_inform:ENDP
IF ans$="M" THEN exitflag=1
PROCcolour_change
IF ans#="R" THEN PROCcircle_title:PROCcircle
:PROCelastic_inform:ENDPROC
L.2570 ,2600
2570
2600
IF ans#="BD" THEN PROCline_title:PROC1ine
IF ans$="P" THEN PROCpoint_title:PROCpoint
Celastic_inform:ENDPROC
2620
IF ans#="S" THEN PROCsave_screen:PROCelastic
_inform:ENDPROC
2630
IF ans#="L" THEN PROCload screen:PROCelastic
_inform:ENDPROC
2640
2660
2670
2680
2670
2710
2720
2730
2740
2750
2760
ENDPROC
DEF PROCpoint_title
PRINT TABC1,19 SPCC79)
PRINT TAB(15,1)>3;"Point Mode"
ENDPROC
DEF PROCpoint
REPEAT
togf 1] ag=1 :PROCdr aw
ans#=INKEYS( 1)
PROCcol our_change
IF CADVALCOXAND 3)9<>0 THEN PLOT 69,x,y»:REPEA
T UNTIL CADVALCO2 AND 3)9=0
2770
2780
27970
2800
2810
2820
2830
2850
2860
2880
2890
2700
2710
2920
2930
UNTIL ans#="M"
PROCreinform
ENDPROC
DEF PROCline_title
PRINT TABC(1,1>9 SPCC79)
PRINT TAB(15,1>;"Line Mode"
ENDPROC
DEF PROCline
x1=x:yl=y:REPEAT
PROCcalc_xy:PROCcursor(ol dx ,oldy)
x2=xiy2=y:GCOL 3,colour:MOVE x1,yx1:DRAW x2,y2
anst=INKEY#¢ 1)
IF CADVALCOXAND 3) THEN ans#="M"
PROCcursor( x,y) :01 dx=x:o0ldy=y
IF ans#<>"M"THEN GCOL 3,colour :MOVEx1,y»1:DRA
W x2,y¥2
2940
2950
2960
2970
2990
3000
3010
3020
3040
3050
3060
3070
3075
3080
3090
3110
PROCcolour_change
UNTIL ans#="M"
PROCreinform
ENDPROC
DEF PROCfil1]_ title
PRINT TABC1,1> SPCC?9)
PRINT TABC15,1)3"Fill Mode"
ENDPROC
DEF PROCfi11
FOR I=1 TO 3
REPEAT
togf 1] ag=1:PROCdr aw: ans¢$=INKEYS$( 1)
IF ans$<>""THEN PROCcol our_change
UNTIL CADVALCODAND 3)<>0
PLOT 69,x,y:x¢I=xiry(C Il =y
REPEAT UNTIL CADVAL(COQ2AND32=0:REM AWAIT BUTT
ON OFF
3120
3135
3140
3150
3160
3180
3190
3200
3210
3230
3240
3250
NEXT I
PROCcursor¢(x,y):REM cursor off
FOR’ T=1 T@G=2:PLOT 694x011) ,rt( 1) tNEAT 1
PLOT “83,x >
ENDPROC
DEF PROCcircle_title
PRINT TABC1,1) SPCC ?9)
PRINT TABC15,193;"CIRCLE MODE"
ENDPROC
DEF PROCcircle
PROCI1ine
GCOL 3,colour:MOVE x1,y¥i:DRAW x2,»2:REM RUBO
UT LINE
3260
3270
3370
3380
3390
3400
3410
3420
3430
3435
3440
GCOL O0,colour:REM BACK TO NORMAL PLOT MODE
PROCcursor(x2,¥2)
radius=SQR¢(x2-x1)*2+Cy2-y1)*2)
MOVE xltradius,y1
FOR angle=0 TO 2*PI STEP Q.1
rx=xl+tradius*COS¢angle)
ry=yltradius*SINCangle)
DRAW rx ,ry
NEXT angle
DRAW xitradius,yl
ENDPROC
THE HOME COMPUTER ADVANCED COURSE 1307
The Plot Thickens
There are two kinds of printer
plotters — drum and flat bed.
Although drum plotters are more
common, the more expensive
flat bed plotters, like the one
shown here, are better for
precision work. This is because
the paper is held steady and the
pen can be accurately
~ positioned. Drum plotters are
less accurate because the paper
can slip on the carriage
PET
The Commodore PET, or Personal Electronic
Transactor, represents one of the milestones in the
development of the microcomputer. Its
appearance in 1977 can be considered as one of
the major events that directed computing beyond
the realm of data processing departments and
electronics enthusiasts towards the ‘dream’ of a
computer in every home.
The precedent established by the PET was that
it was not only fully assembled with its own casing,
but it also had a built-in cassette deck and monitor,
as well as BASIC resident in ROM. You had only to
plug in the machine and power it up in order to use
it. Previously, microcomputers were available only
as a kit or with minimal assembly at best.
PICTURE PROCESSING
Also known as ‘image processing’, picture
processingis the digital analysis of signals supplied
from a television camera or similar device which,
after processing, can be displayed either on a
television or video monitor or as a printout. The
amount of detail displayed is a function of the
definition (resolution) of the image, of which each
point is referred to as a pixel. So, the greater the
number of pixels there are within the picture, the
higher the definition is said to be.
The image received from the camera is held as a
two-dimensional array within the computer. The
individual pixels can be stored within the array ina
number of different ways depending on the
program used. At the simplest level, however, the
information stored can merely state whether that
pixel is on or off (whether or not light is falling on
the corresponding point of the camera image).
However, other information can be stored and
manipulated within the computer, and this may
1308 THE HOME COMPUTER ADVANCED COURSE
specify colour or shades on the ‘grey scale’ (the
relative contrast of the pixel compared with others
in a monochrome image).
Picture processing is finding an increasing
number of applications in the television and
satellite communications fields. These range from
the stunning effects now being produced in films
and videos to improving the definition of the
sometimes hazy pictures received from space
probes. Picture processing is also being applied in
the field of artificial intelligence where it is used
with the emerging techniques of pattern
recognition. In this area the emphasis is not merely
on programming a computer to produce a
required display, but rather enabling it to interpret
an image and attempt to understand its ‘meaning’.
PLASMA DISPLAY
The method of plasma display, which 1s
increasingly being used in computers, is
characterised by an orange or red light emanating
from characters similar in design to those used on
LCD displays. A plasma display is constructed by
trapping a gas between two materials, one of
which must be transparent (to enable you to see
the display). At various points around the display,
electrical lines are connected to form a matrix, so
that when a current is applied to them, the gas will
become charged and emit light. In this way,
various combinations of currents through the lines
can produce characters or designs.
For the most part, plasma displays are made to
produce single characters, although it is possible to
build larger displays. However, the costs of these
tend to be too high, especially since their functions
can be performed just as well by devices
incorporating cathode ray tube technology.
PLOTTER
A plotter is an output device that receives digital
signals from a computer and translates them into
graphical form, usually onto a piece of paper with
a pen. The movements of the plotter can be
directed along either the x and y axes, or else
vertically, to raise and lower the pen. Of the two
basic kinds of plotter, the first is known as a ‘flat
bed plotter’, onto which a single piece of paper is
laid. The pen is mounted on a bar that moves along
one axis, while the pen itself moves along the bar,
thus able to plot on any given point within the
limits of the hardware.
The most usual type of flat bed plotter consists
of a board to hold the paper and moveable arms to
position the pen, but a recent imnovation
beginning to gain popularity is the robot, or turtle
plotter. With these, the pen mounting travels
freely over the paper on a motorised device. This is
controlled by the computer, usually via a ribbon
cable. Perhaps more familiar to home computer
users, a drum plotter feeds a continuous roll of
paper onto a drum, much like a typewriter. The
pen mounting moves across the paper, along the x
axis, while vertical movement, along the y axis, is
achieved by the drum rolling the paper through.
Sf A
———4y-
With its new 130XE micro Atari makes
virtue of compatibility both with its XL
predecessors and the familiarity of its
graphics, sound and other features. What
places it in a challenging position in the
market, however, is its seductive styling and
RAM capacity of a full 128 Kbytes,
available at a very competitive price.
Despite the fact that Atari was one of the early
pacesetters in the home computer market, the
early 1980s were not good years for the
corporation. In 1984, after suffering heavy losses,
it was taken over by the former head of
Commodore, Jack ‘Tramiel, who set about
reversing the fortunes of the ailing computer giant.
His policy of marketing computer technology at
the lowest possible price was soon evident in the
shops when the prices of the 600XL and 800XL
micros were dramatically reduced in time for the
Christmas market.
In itself, this move was not enough to reverse the
trend. Atari was faced with a problem common to
many computer manufacturers: low sales leading
to lack of software support, which stifles sales even
more — meaning that funds are lacking for further
investment in new machines.
It is wronic that Jack Tramiel was the person
most responsible for placing Atari in this position.
His aggressive marketing of the Commodore 64
led to Atari being almost wiped out by its rival. By
the beginning of 1985, the situation was changing
dramatically. Commodore was faced with falling
sales and the failure of its Plus-4 to make a major
impact on the market, while Atari announced a
host of new products. The first of the new
machines is the Atari 130XE, a computer based
around the 6502C processor.
The 130XE is essentially the same machine as
the basic Atari eight-bit computer, which has been
around in one form or another since the beginning
of the 1980s. The main difference between this
machine and earlier Atari computers is the sleek
new styling and the massive amount of on-board
memory: the Atari 130XE boasts a full 128
Kbytes of RAM.
The casing of the computer looks very different
from that of its predecessors. The light grey plastic
outer shell has the elegant design the public has
come to expect of a modern computer, with
rounded lines and wide, sculpted keys that allow
for ease of typing. The keys have a slightly better
travel than that offered by previous models, with
the added advantage of not rattling as you type.
In common with the other 6502-based Atari
micros, the 130XE has five pre-programmed
function keys. However, unlike the previous
CHRIS STEVENS
Getting Back Into Shape
The Atari 130XE is the first
product to be launched under
the company’s current
president, Jack Tramiel. With
128 Kbytes of RAM on board, at
a price that compares favourably
with many computers with only
half the memory, Atari hopes
that this will be the machine that
will revive the company’s
fortunes
THE HOME COMPUTER ADVANCED COURSE 1309
models, these have been placed above the main
keyboard rather than on the right-hand side. They
are now moulded in the same grey plastic as the
rest of the casing and are designed in the stylish
form of parallelograms rather than squares. ‘These
are a great improvement on those of the XL series,
which were metal and had a distinctly unsure and
wobbly feel.
INTERFACE CONNECTIONS
The interfaces fitted on the rear and right-hand
side of the new Atari also hold few surprises. On
the side are the expected nine-pin D-type
joystick ports first used by Atari and since adopted
by almost everyone else. At the back of the
machine is the 13-pin serial control port that 1s
used by Atari to fit and daisychain Atari
peripherals such as cassette decks, disk drives and
printers. To the right, set into the casing, are the
cartridge and expansion interfaces. The cartridge
port enables the machine to run the large quantity
of highly rated Atarisoft games cartridges, such as
Pacman and Galaxians, that have been one of the
strengths of the company over the years.
The expansion port, however, is something of a
departure from the previous standard. Earlier
Atari computers had a 50-way edge connector as
an expansion bus. The new machine has a much
smaller 14-way bus fitted as a cartridge connector.
The remaining interfaces on the 130XE are the
composite monitor socket, an RF jack plug for
television sets and the standard Atari power-
supply socket.
To keep costs down the 130XE has no on-
board modulator to allow it to work effectively
with British standard televisions, but has the US
standard instead. Thus, to allow UK sets to be
used, the aerial lead from the computer to the
television runs via a small box that modifies the
signal into one that will correctly process the
television display.
Styling and compatibility are all very well, of
course, but Atari applied these to the XL series
without conspicuous success. What makes the
130XE different from earlier machines and what is
its obvious selling point is the availability of a large
memory capacity at such a low price. An eight-bit
microprocessor can, of course, address only 64
Kbytes of RAM at a time. To address twice that
amount the computer has to make use of the
process known as ‘bank switching’. Using this
technique, the computer can look at a ‘window’ of
64 Kbytes among the total 128 Kbytes. While the
technique is not perfect, and although the extra
instructions necessary to switch from one bank of
RAM to another result in somewhat slower
retrieval speeds, it does allow an eight-bit micro to
address more memory than would otherwise be
possible.
Actually, the bank switching technique is more
common than you might think. Both the Oric
Atmos and the Commodore 64 have more than
the normal complement of 64 Kbytes on board,
and both use bank switching as a means of making
1310 THE HOME COMPUTER ADVANCED COURSE
Expansion Port
The single departure of the
130XE from previous Atari
peripheral ports is the
expansion slot
RF Socket
The 130XE RF control is
configured to the US standard.
Therefore, the aerial lead is
fitted with an additional box
which contains the logic to
conform to the UK standard
RAM Chips
The 128 Kbytes of RAM are
stored in these two banks of
8- Kbyte chips
Memory Control Chip
Memory management and
bank switching routines are
contained in this new chip,
which has been christened
‘Freddy’
Graphics Chips
The ANTIC and GTIA chips
control the computer's screen
graphics
CPU
Like all previous Atari
machines, the 130XE is based
around the 6502 processor
Cartridge Port
The cartridge port enables the.
computer to take advantage of
the wide range of Atari
software
Peripheral Port
interface
Joystick Ports
PIA Chip
input/output control is
managed by a 6520 chip
Sound Chip
The ‘POKEY’ chip is
sound capabilities of the
130XE
Atari peripherals, such as disk
drives and printers, can be
daisy chained together with
the computer via this serial
The computer is fitted with a
pair of joystick ports which,
naturally, are Atari standard
responsible for the four octave
full use of the available memory.
What is unusual is the price of the 130XE.
Although more expensive than the Sinclair
Spectrum and Acorn Electron, the 130XE is
considerably cheaper than the list prices of the
BBC Micro and the Commodore 64. In order to
provide a 128 Kbyte computer at that price and
produce a profit, Atari needs to have cut
production costs considerably. To an extent, the
nature of the machine itself will have kept the price
low: the 130XE is essentially a revamped
machine, which means that development costs
have been kept to a minimum. The main
economies have taken place inside the machine.
CUTTING COSTS
The memory area comprises 16 eight-Kbyte RAM
chips. The production cost of these chips, which
are no longer considered the product of ‘leading
edge technology’, has fallen dramatically in the
past few years and this is reflected in the price.
Another way of reducing costs is to keep the
number of components on the board to a
minimum. Although many of the chips on the
earlier XL series have found their way into the
130XE in order to maintain compatibility, the
printed circuit board is exceptionally well laid out,
looking considerably less cluttered than many
machines with only half the memory capacity. -
Finally, Atari has invested heavily in automated
assembly plants, of which the 130XE is the first
product. All the components on the board are
soldered on by machine.
As none of the sound, graphics and Basic ROM
chips have been altered in essentials, the computer
is, for the user, exactly the same as the earlier
models, with the high-performance sound and
graphics with which Atari has become associated.
One major change of benefit to the user is the
manual. The handbooks that came with earlier
models tended to be oversimplified to the point
where they appeared to be aimed at children. The
BAsic tutorial is much improved and the company
has given some technical specifications in the
appendix. However, for a fuli explanation of the
dialect, one still needs to purchase the Atari BASIC
Reference Manual.
Although the Atari range of micros was sorely
in need of upgrading, the arrival of the 130XE is
something of a puzzle. The extra 64 Kbytes of
RAM provide a lot more memory for the
programmer and yet there are no programs
available as yet to take advantage of it, even
allowing for compatibility with the Atari software
already on the market. Normally, one would
expect the launch of such a machine to be a
preliminary manoeuvre before a serious thrust at
the small business market. However, the new
Atari management has consistently denied any
such intention. Perhaps the real reason for the
launch of the 130XE is that Atari intended to pre-
empt the launch of the Commodore 128, a
Commodore 64 compatible machine that also has
extra memory but is priced much higher.
ATARI 130XE
£169.95 inc VAT
350 x 233 x 63mm
6502C running at 1.79 MHz
128 Kbytes of RAM, 24 Kbytes of
ROM
40 x 24 text display, 320 x 192
pixels (high-resolution) with 256
colours available
Cartridge port, TV jack, composite
_ monitor socket, two joystick ports,
serial input/output port,
expansion interface
Atari BASIC, LOGO, FORTH, PILOT
62 keys, including five pre-
programmed function keys
The manual gives a full
explanation of Atari BASIC,
although the tone is still rather
oversimplified. Explanation is
given in the appendix of the
interface configurations and how
to ‘bank’ the other 64 Kbytes of
RAM
“The 130XE has the advantage of
being able to trade on Atari’s
traditional strengths, yet
possesses an additional 64 Kbytes
of RAM that can be used as a
‘silicon disk’ for fast storage and
retrieval
- The computer does not solve
Atari's basic problem of lacking a
large third-party software base.
The fact that the 130XE is
essentially a revamped version of
a machine that has been around
for some years may mean that it
will not generate the interest in
customers that the company
needs
THE HOME COMPUTER ADVANCED COURSE 1311
~ SEARCHING
LOOKS
The flow of program control in
PROLOG does not follow the
familiar sequential pattern used in
languages such as BASIC and
PASCAL. We discuss PROLOG’S tree-
like search procedure, highlighting
the general steps between defining
a program’s objective and
achieving it.
- While languages like Basic and PASCAL
have sequential flows of control, with
control passing from statement to
statement in strict top-to-bottom order
(unless a loop or a GOTO interrupts it),
| the flow of control in proLoG takes the
form of a depth-first search through the
program clauses.
To understand this more fully, think
of the program as a tree, with the goal
(proposition) to be proved at the root
and all the sub-goals as choice-points
where the lower branches divide. There
are many ways to search a tree like this
but the method used by PROLOG is to
take the leftmost branch and follow it
down as deeply as possible. As it tries
each branch, it marks its trail, and when
it reaches the bottom and can go no
further, it backs up to the nearest
choice-point, taking the leftmost
branch that it has not already taken,
and carries on down from there. In this
way, the system will eventually explore
every path through the tree and thus
will have tried every possible way of
proving the topmost goal.
An exhaustive depth-first search is
guaranteed to cover all paths, but could
be a lengthy business. In fact, PROLOG
manages to save itself some work. A
PROLOG Clause Is, aS We Saw in Our
queue(X)
previous instalment, a rule which says
that the head goal is true if all of the
subgoals are true:
goal:— subgoal1,subgoal2,subgoal3. ..
etc.
and so on. This is probably more easily
understood if we write it as:
IF subgoal! is true
AND subgoal2 is true
AND subgoals is true
AND etc.
THEN goal is true.
Because the subgoals are ANDed
together, the whole clause will fail if
any one of them cannot be proved. So
PROLOG works through the subgoals
from left to right and, if it fails to prove
one, will stop at that point and not
bother with the rest.
Backtracking can make programs
behave in a way that makes the order in
which the code is written almost
irrelevant. However, the advantage is
that the flow of control, instead of
being a major concern as it is in BASIC,
has only a minor importance, leaving
you to concentrate on the logical
structure of your problem.
The emphasis that PROLOG places on
a ‘declarative’ statement of the problem
does not mean that you cannot see
your programs as behaving
procedurally. The pRoLoc clause:
martian(X):— no. of limbs(X.7), no. of
heads(X.2), can program in(X,cobol).
can be read declaratively as: ‘X is a
martian if X has seven limbs, two heads
and can program in CoBoL’. Its
procedural reading would be: “To
prove that X is a martian, first prove
1312 THE HOME COMPUTER ADVANCED COURSE
Question Time
This flow-diagram shows
PROLOG in action, answering a
simple query posed by the user.
Note that the variable X is given
two different values during
program execution, yet each
value is retained. This is
possible because PROLOG e
treats variables as local to each
separate invocation of a clause
that it has seven limbs, then prove it has
two heads, then prove it can program in
COBOL’.
LISTS AND RECURSION
PROLOG 1s not as strongly ‘typed’ as
most other languages. Nor is it fussy
about the data types of arguments
within its terms. So the term
pred(Argument) could be used at
different times with the variable
Argument set to an integer, an ‘atom’
(such as marty, venusian, d24, and so on),
or a list. However, PROLOG treats its
data types in different ways, allowing,
for instance, numbers to be
arithmetically manipulated.
Anyone familiar with LoGo or LIsP
will have come across the list data type
before and the special ways that are
used for manipulating lists. A PROLOG
list is written enclosed in square
brackets with the list elements
separated by commas. So
[apple, pear, banana] is a list of fruits,
[a,f,e,g,1,X] 1s a list of letters and so on.
To get inside lists, PROLOG lets us take
them apart one element at a time by
removing the first element. The
notation [Head | Tail] describes a list with
the element Head and the rest of the list
in the list Tail. Applying ‘I’ to our list of
fruits will give us [apple | [pear,banana]].
As you can see, lists can have other lists
as members. As a special case, if we
take the list [280] with one element and
break it up with |, we get [z80 | []]. The
list in the tail is [], which represents the
empty list.
PRo.oG allows recursion, which, in
fact, is the normal style for a PROLOG
program. A recursive definition is one
that defines something in terms of
Lila
itself. In PROLOG, we might write:
queue([person)]).
queue([person | X]):— queue(X).
When we have two or more clauses, as
we have here, which have the same
head, they are known as a procedure.
The queue procedure has a clause
defining a queue as a list having one
element, person. It then has a second
clause informing us that a queue could
also be a list with the element person at
the head and with a list called X as its
tail. We then see from the right-hand
side of this clause that X must itself be a
queue.
If we give PROLOG a goal such as:
queue([person, person, person)]).
asking it to say whether the list
[person, person, person] is a queue, it first
looks for a clause in its database to
match our goal. The first one it will find
is queue([person]). This does not match
because the lists are not identical, so it
will scan down to the next clause:
queue([person | X]):— queue(X). This
does not match either and it fills in the
values of the variables like this:
queue([person | [person,person]]):—
queue([person,person)).
To show that the head goal is true, it
must show that the subgoal is true. So
PROLOG takes queue([person,person]) as
its goal and begins scanning down the
clauses from the top to find a match.
Again, queue([person]) does not match,
but the second clause does, giving:
queue([person | [person]]):—
queue([person]).
An important point to note here is that
the variable X, which on the first run
through was set to [person,person], has
now been set to [person]. Yet the first
value of X has been retained.
This is possible because variables are
local to each separate invocation of a
clause; thus every call to queue([person |
X]) can be thought of as using a separate
and unique variable. In PROLOG, there is
no such thing as a global variable.
Having matched the head of this
goal, to prove it we need to prove the
subgoal on the right of the :— symbol,
which is queue([person]). Another scan
of the clause-base is begun and this
time a match is made with
queue([person]), which succeeds straight
away and means that
queue([person, person]) (our previous
goal) is now shown to be true. This in
turn means that queue([person,
person,person)), the original goal, is true.
KEVIN JONES
The queue procedure shows us a | |
number of important things about
PROLOG. For instance, the ordering of
the clauses can be crucial. (Try placing
the two clauses in the opposite order
and see what happens when you try to
prove a goal.) It also illustrates how
giving extra clauses is the same as
giving alternative ways of proving a
goal, just as if we had used a logical OR
between the clauses. This means that
PROLOG does not need an OR operator
— although most implementations
provide one.
KEVIN JONES
THE HOME COMPUTER ADVANCED COURSE 1313
The final instalments of our New World
simulation game project are devoted to
considering the Basic flavours that will allow
the game to run on the Spectrum, the BBC
and the Amstrad micros. This article focuses
on Spectrum flavours and includes the first
section of the complete common listing.
The program was written on the Commodore 64
but uses minimal Basic where possible. Problems
in converting the program to run on the Spectrum
lie in two main areas: first, the Spectrum will allow
only single-letter variable names to be used with
arrays or FOR... NEXT loop counters. We provide a
conversion table here. Second, Spectrum string
handling is unusual in that LEFTS, RIGHTS and MIDS
are not available, although each has a Spectrum
equivalent. Spectrum owners should refer to the
flavours given with each module for these
conversions. In addition occurrences of PRINT
CHRS(147) should be replaced by CLS and lines that
wait for keypresses of the form:
<line no> GET IS:IF IS=“” THEN <line no>
should be replaced by:
<line no> LET IS=INKEYS:IF IS=“” THEN GO TO
<line no>
In the next instalment we will give the second
section of this full program listing.
1314 THE HOME COMPUTER ADVANCED COURSE
ae
Bisa | “Sasa
BOSUBS 248
FORT=17TUS
Ch i.
tin & &
mS it}
TFORS La =1THENS#="BOTTLE OF MEDICINE #":GOTO?735
S$="BOTTLES OF MEDICINE*"
PRINTOARS 13: :GOSUB?1h6
GOSUBP2H8
IFOAS 2)=8THEN 736
TFOAC 2)=1THENS#="GUNe" :GO0TOr45
oJ oJ J ~J
ba IFCCS T =H T HENS a
S50 PRIBITELC TD s
446 PRINTCECT2 :
S62 IF CECT ISLTHENPRINT" "“:GOTO46S8
S64 PRINTS"
4438 GOSUB?2H
3° NEXT
474 GOSUBS2H4a
475 PRINT: S¢="AHO THE FOLLOWING FROWISIONS:s":G08u
Bia
S56 GOSUBPSAA
S85 FPORTS=i 7o4
S54 ITFRAtTI=s8 THEN 714
S75 PRINTPACT) :US¢T):"S OF ":
FRR PRIPITRSCT 3
*HS GOSUBS2aG
71H NEXT
f1S GOSUBS2H4u
fe PRINT: S#="YOU HAVE ALSO GOT:#" :GOSUBS1 44a
f25 GOSUB?2a8
YSh [TFOAC 1 =a8THEN?4a
aS
B4
5
&
A
3
jm
‘ St=" GUNS"
& PRINTORS 2); :G0SUB71 68
e GOSUB?2H8
é TFOARS S)=8THEN Sse
~]
TFOR*« 33=1THENS#t="BAG OF SALT#®":GOTO?SS
S$="BAGS OF SALT®="
PRINTOAt 333 :GOSUB?1 48
GOSUBS 246
TFOAR! 4)=8@THEN? 7a
1 Som
of Oo ICN inion et hee Bb i to
hein eb oi Sm Opi eo on tt
=.
e TFOR¢ 4¢=17THENS#="BALE OF CLOTH*" :GOTOFS5
764 St="BALES OF CLOTH#"
fé5 PRINTOAC 4S) ;:GOSUBS1 668
#64 GOSUBS24H
f?@ TFOAtSI=8THEN? Se
?fS ITFOAtSI=1 THENS#="KNIFE®" :G0TO?F 75
ff4 St="KNIVESS"
??S PRINTOACS) ;:GOSUB?1 Aa
fe é GOSUBS2b8
#88 ITFOAt6)=6THEN? 74
PSS LFOARC 4)=1THENS#="JEWEL®" :GOTOFSsS
@S4 S¢=" JEWELS*"
°S5 PRINTOAC 69; :GOSUBS1 ab
786 GOSUBS2H4
°F GOSUBS 244
??2 PRINT:PRINT" YOU HAVE "“sh0O:" GOLD PIECES LEFT"
@?6 GOSUBS2h48
??? St="PRESS ANY KEY TO START VOYAGES"
“78 GOSUBS1 48
2? GETIS:1F Il¢=""THENF SS
SHB WT=6 :REM ZEROISE WAGE TOTAL
S61 HE="N" :REM HALF RATION INDICATOR
S862 DIMHR( 4) :HROLI=1:HR¢ 2)=1:HR¢3)=1:HRi43=
Wk 1
GOUSUB4SHBH: REM CREW STATUS REPORT
GOSUB42@6:REM PROVISIONS REPORT
BOSUBS3H8:REM OTHER GOODS REPORT
GOSUBF 208 :PRINTCHRS: 1473
FRINT: PRINT :PRINT
S43 S8="IT 15 ESTIMATED THAT THE VOYAGES" :GUSUB?1lUa
S44 PRINT"WILL TARE A FURTHER" :INT¢ JL-WkK+1): "WEEKS"
B45 GOSUB? 26H
846 PRINT: S#=K#:GOSUB?S1 aa
S47 GETIS:1IFI¢=""THENS4?
854 GOSUBSHBH:REM CHECK WAGE BILL
855 GOSUBSi6H: REM ISSUE RATIONS
$66 GOSUBSShb
61 REM GO TO GENERATE RANDOM EVENTS
GOSUBSS8@:REM GOTO MAJOR CONTINGENCY
IFHR* S9=.5ANDRND¢1)¢. STHENPRINTCHRE( 147) :GOSUB
J
=
ee
pica
REM ALBATROSS IF SHORT OF MEAT
GOSUBZ 2H
GOSUB S36: REM ENOG-OF-WEEK REPORT
WIKSLIK +1: 0FWK <= JLTHENS 25
REM ARRIVAL AT NEWWORLD
GOSUB1 Gb
GOSUBIaB Ta
GOSUBIaSHE
GOSUBIGSHEe
“J 7) oS
00 oO
Oh Whee eA © co on on
“oo 0 oo 0 oo CO co oo a
“Oo ~O MO ~O
END
1@8@ PRINTCHR#(147):PRINT" STAGE 1 - HIRING CREW"
141@ PRINT" ---------------------- ‘
i@i2 PRINT
1815 GOSUBSza4
SIMULATION GAME/PROGRAMMING PROJECTS
PRINT: PRIWT"CRELD TrPES GUSILGABLE:"
GOSUB? 2a
FPRICT
PRINT" TYPE DESCRIPTION WAGES PER WEEK"
PRIWT"---- ----------- -—----—---—----—-—-— "
FRIMT " SAILOR
PRINT " DOCTOR
PRITIT " MECHANIC
PRItHT" HALT GATOR
FPRICIT" Cook
GOSUB? 2H
PRIWT PRINT :PRIWT
St="ENTER CRE TYPE RPEQUIRED( 1-5) #" :GOSUB?1 46
li2e Sé#="CF “F° TO FINISH HIRING*" :GOSUBS1 ab: PRINT
:INFPUTIS
1125 ElSURLt lS)
1i28 [FLEFT#¢ 1,1 .="F"THENPRINT:FRINT"“EMD OF CREM
HIRE." :GOSUBS2h6:G0TO1 Siu
1138 IFCT?}0ANDCT<4THENI 156
liso PRINT PRINT
1146 PRINTI#¢;:S¢=" IS NOT A CREW TYPE®" :GOSUB?1 64H
1142 GOSUBS266
1145 S#t="FPLEASE ENTER AGAIN"
1146 GOSUBY146
li4? GOTO1 S66
1156 PRINT: PRINT
11355 CN=CN+1:REM CREW HIRED SO FAR
1156 TS*CN,12=AT:REM CREM TYPE
115” TS¢CCN,2)=196:REM STARTING STRENGTH
1158 WSLIT+WG0CT3 : REM TOTAL WAGES
115% COCCTISCOCCT3+1:REM CREW TYPE COUNT
i146 St="CREL SO FAR:"
1178 FORT=i1TU5S
LiS@ PRINIS#:Clito;" “Ceca;
1185 I[FCCCT) F1ORCCeTISe8THENFRINT"S" :GOTOLIS=
L188 PRINT? &
liss S¢=" .
live NEXT
11735 FRINT: PRINT"TOTAL WEEKLY WAGE BILL ";LIT
1266 TFCH=CM-1THENPRINT :S¢="0NLy ONE MORE CREWS" :6
OSUBFIBE :GOTOL2S5
1262 TFCN=CMTHENPRINT :S¢="
UBS1 ae :G0TO1L S16
Si MS mH
“Jo. Uf ts ba bl ni
Woo LA mm Hm em me LA ee
GOLD PCS"
GOLD PCS"
GOLD PCS"
ms he he
Cy oS on on em
Peep
Oh fe fo hae
Ss et Eom
Moe @ ow
—
—
SHIP WOW FULL! I" sG0s
l27o REM
1366 GOTO1L815
Tsl6 PRINT: S#=k¢:G0SUBS1G8:PRINT: GOSUBS 24
1326 GETI¢:1TFIS=""THENI S26
17? > RETURN
2606 PRINTCHRé( 147)
2618 S¢=" STAGE 2 - PROVISIONINGs"
2615 GOSUB?I OG
2620 S¢=" ----- ee a
2025 GOSUBS1aa
2636 GOSUBS200:PRINT
264@ PRINT"YOU’VE HIRED & CREW OF ";CN;"."
2645 GOSUB?2h6 :GOSUBS 2H
2656 FORT=17T04
2055 PRINT
2666 PRINT"EACH CREW MEMBER WILL NEED "
ebfO PRINT RI LEAST “:PNNC TOs" 8 lect;
2675 IFPNCTI=1THENPRINT" “::GOTO2685
2686 PRINT"S":
2065 PRINT" OF ":Pec To
e2eUSe PRINT AT ;PC Clos" GOLD PES PER ( - User Ts
2676 PRINT"FOR EACH WEEK OF THE JOURNEY."
2475 GOUSUBS246:PRINT:GOSUBS 244
2106 PRINT HOW PANY “Uti lo: "S OF “sPecT >
2114 S$="D0 YOU WANT TO BUY#" :GOSUB?1 64
2120 PRINT
2138 INPUTIS
2146 PACTISVALC I$) :GOSUBS2H4
2156 ITFPACT) 2° (CNSS*PNCT)9-1) THEN2268
2166 IFRPACTI=@THENPRINT"IF YOU DON’T BUY ANY" :GO0TO
2176 PRINT"IF YOU ONLY BUY "sPACT) sUS¢T):
2175 IFPACTI=1THENPRINT" OF" :GO0TO21 S68
21/76 PRINTS OF"
2186 PRINTPS(T):;"," :GOSUBS206
21748 PRINT"SOMEONE MIGHT GET ":
22648 St="HUNGRY"
2216 IFT=4THENS#="THIRSTY"
2226 PRINTS®;"!":GOSUBP2ha
2236 S¢= "DO YOU WANT TO TRY AGAIN" :GOSUBS14a6
22496 INPUTPS:P#=LEFTS¢P#, 1)
2242 [FPS<2"V"ANDPS<>"N" THEN2236
22935 [FP#="N"THEN24466
2250 PACTI=8:T=T-1:G0TO2416
2266 TFPACT)*®PCCT? >MOTHEN22 76
2269 GOTO2404
22/76 Sé="VYOU DON’T HAVE ENQUGH MONEY FOR": GOSUBS1@ 4
PRINTPACT 3
PRINTUS¢7T):"S OF ";P#c1T> :GOSUBS 286
S$="PLEASE TRY AGAIN®" :GOSUBS1H4:PACT)=8:T=T-
s0TO249168
MOSMO-¢ PACT) #POCCT 32
PRINT :S#="PROVISIONS SO FAR: *" :GOSUB?1 a6
BOSUBS 244
FORTT=17T04
FRINTPACTTs sUBCTT) s
IFPACTT2=1THENPRINT" OF ":;:GOTO2446
PRINTS OF |;
FRINTPS<TT 2
BOSUBS SHE
mS
fo ha fal
Newer ee aos wo
1
Wm eS im oA RY
Pah ha bo po ho Bo Ro Bo Po oh po
BHEhHREBRLREE Es
THE HOME COMPUTER ADVANCED COURSE 1315
PROGRAMMING PROJECTS / SIMULATION GAME
2466 NEXT
2496 PRINT"MONEY LEFT = "iM0;" GOLD PIECES"
2485 GOSUBS200:GOSUB? 208
2490 MEXTT
2500 GOSUIIBSZG1:PRINT:S#="END OF PROVISIONING#":G05
UBS190:GOSUB92a6
S510 PRINT: S$=K$:GOSUB?1GG:PRINT: GOSUB?2a8
2520 GETI1$:/= =." 'THENS5ea
2999 RETURN
3008 PRINTCHRS(147): REM STAGE 3
3082 GOSUBS2a8
2985 PRINT" STAGE 2 - OTHER GooDS"
301@ PRINT" 9 ------- 0 we :
3028 GOSUBS200
3425 PRINT
S030 S$=<"THERE ARE OTHER THINGS THAT Mays" :G05UBFiaa
S¢="BE USEFUL ON THE VOYAGE, FOR #":G0SUB7188
S$="EXAMPLE MEDICINE AND TRADING #";:GOSUBs1ia8
GOSUB? 286
Om cm tsa Uo be on
ihe Sue on ee Amo om cnt
arm Se She mS we ee ce
MUN b bh ewww oo
ds hg
CA oS oe
s) Of mr CO St LA Ge CA
Jo 0) ta og CO oo 0 Oo Oo oo to od 0 to oo mo hdl go he oo
ee ee el ee eC
4008
Agia
«4a28
4025
4634
44035
4a4a
4045
4455
4068
4204
4205
4206
4z14
& PRINT "MONEY
St="THE
St="OU May ALSO NEED WEAPONS. =":GOSUBSiaa
GOSUB? 286 :GOSUB? 204
FORT=1TO4
PRINT
PRINT" "s;O8¢T):
S$=" COSTS*" :GOSUB?104
PRINTOCST):
PRINT" GOLD PIECE";
IFOC<T?=1THENPRINT" ":GOTO269
PRINT"S"
GOSUBSZ00
S#t="WOULD YOU LIKE TO
INPUTPS:PS=LEFTS¢P$,12
BUY (Y OR Noe" :GOSUBT1E&
TFPS<>"¥"ANDPS<>"N"THENS@ 75
LFP$="N"THENS1 75
GOSUBF2a0
Sé="HOW MANY DO YOU WANT*" :GOSUBF1a8
INPUTI#
TTSVAL (183
IFOC’T)*TT >MOTHENS1 58
GOTOS1 40
S$="YOU DON’T HAVE ENOUGH MONEY *" :GOSUB91 a6
GOSUBS 286
S$#="FLEASE ENTER AGAIN *" :GOSUB?1i 46
GOSUBS 248 :G0TO31 38
MO=MO-COCeC TTT?
OACTISTT
GOSUBF 204
PRINT
LEFT = ‘M0
GOSUBS264:NExXT T
GOSUBS264:PRINT :FRINT
S$="END OF STAGE 3*" :GOSUB?1 66
GOSUBS 266 :PRINT
S#=kt : GOSUBF1 AG
GETI¢:1TFIé=""THENS246
RETURN
REM CREL STATUS REPORT
PRINTCHRE( 1442
St$=—" CAPTAINS LOG*s" :GOSU
Se" #" »G05
GOSUBS 246
PRINT"AT THE START OF WEEK" sWk
STATE OF THE CRED I15:*" :GOSUBS1 44
GOSUBS2ZHa:PRINT
PRINT
FORT=17TO14
IFTStT.13=HTHENSI1&
PRINTCS¢(TS¢T, 1333"
LFTS¢T , 2)=-79?T HENS#=" DEAD
IFTS(T, 23> 757THENS#="VERY HEALTHY?) #" :GOTO46??
IFTS¢t¢T , 2) +S58THENS#="HEALTHY)#" :GOTO4a 9?
TFTS(T,2)>25THENS#="SICK ! jJ#" :G0TO48e?
Sé$="WVERY SIEK ') ae”
GOSUBSI A: GOSUBS Zhu
NEXT
GOSUBS Zhe: PRINT
LJILJ=8
FORT=17T0OS
WhJslild+ § Ee To ele T a3
NEXT
St="WAGE BILL FOR THE WEEK#" :GOSUB?1 ab
PRINTWL;"GOLD PIECES"
GOSUBS Zu
WTSWIT +liu
St="TOTAL WAGES FOR VOYAGE
PRINTHWT:"GOLG PIECES"
GOSUB? 2H 4
PRINT "MOWEY LEFT = "“sM0;"GOLD FIECES"
PRINT :S¢=K$:G0SUBS164
GETI#:IFIS=""THEN41 Fu
RETURH
REM FROVISIONS REFPURT
PRIMTCHRE! 1473 ;
FRINT"AT THE START OF WEEK" sk: GOSUBS 2h
Sé="VOU HAWE THE FOLLOLIING*" :GOSUB?1 a6
S50 FAR: 2#" :GOSUBS186
1316 THE HOME COMPUTER ADVANCED COURSE
Sé="PROVISIONS LEFT: *" :GOSUBS148
PRINT : GOSUBY 264
FORT=17T04
TFPACTI=BORPACT 3=-?779T HENS 244
PRINTPACT) :US¢TI3:"S OF "SPECT?
X=PACTIA¢CCNEPNCT 2?
PRINT" CENOLIGH FOR" ;INT¢xX);" WEEKS?"
GOSUB? 268
MET
PRINT :St#=K% : GOSUBT1 46
GETIS:1TFLé=""THENG295
RETURM
REM OTHER GOODS REPORT
PRINTCHRé*147)
H4 PRINT"AT THE START OF WEEK" ;WkK:GOSUB?2H8
Age
OSUB
4394
i hd i ho pa
CA i om CA om
238
3246
a are
3278
ig eo: 4 ut
S$="YOU ALSO HAVE:#" :GOSUB?1 46
PRINT : GOSUBS 2848
IFOAC 1 3=8THEN4 332
PRIMNTOAC13::S¢="BOTTLES OF MEDICINE®" :GOSUB?144
GOSUBS 264
TFOAt 2)=8THEN4342
PRINTOA( 2)::5¢="GUNS#" :GOSUBF1 a8
GOSUB?2HH
[FORA S3=8THENS+ S52
PRINTOAC3)::5¢="BAGS OF SALT#" :GOSUB?1 a4
GOSUBS 248
IFO0G* 49=6THEN434é2
PRINTOAC 4d) ;:5¢="BALES
GOSUBS2a4
TFOAC S>=6THENSS72
PRINTOA¢ 5) ::S¢="KNIVES#" :GOSUB?1 46
GOSUBS ZH
TFOAS 49=6THENS386
PRINTOAC 6) ::S¢="JEWELS#" :GOSUB?146
GOSUBS 264 :PRINT
PRINT" YOU HAVE" sMO::Sé="
146
BOSUBS ZB
PRINT :S$=K% :GOSUB?I1 a
GETI#:1FIS=""THEN4SSS
RETURN
REM CHECK WAGE BILL
IFT SMOTHENS&@ 16
OF CLOTH" : G0SUB? 106
GOULD PIECES LEFI*® :G
» GOTOSars
PRINTCHRE(14 7)
PRINT : PRINT s PRINT
St="THE CRED! HAVE HEARD A RUMOUR®" :GOSUB?1 48
St="THAT YOU DON’T HAVE ENOUGH®":GOSUBS16a ©
St="GOLD TO PAY THEM AT THE END#®" :GOSUB?1 48
S$="CF THE VOYAGE.*" :GOSUB?1 646
GOSUB?2H6 :PRINT
St="THEY ARE GETTING ANGRY !!*" :GOSUBF1 an
GOSUBS 208 :PRINT
S$="LET’S HOPE YOU MANAGE TO MAKES" :GOSUB?1H4
S$="4 TRADING PROFIT! #" :GUSUB?1 66
GOSUBF 268
PRINT: S#=K$:G0SUB? 108
GETI#:1FIS=""THENSaSe
RETURN
REM ISSUE RATIONS
PRINTCHRE<( 1472
ISSUING RATIONS#" :GOSUB?1466
Si ee ee ee ee *" :GOSUB?1&6
GUSUBS2H4 :PRINT "VEER" (WK: PRINT
H$= it pl by
FORT=17T04
HRT 3=1
IFPACT?) #@THENS1 58
PRINT"NO “:Pecio;" LEPI! |!" -GUSUBy eee
S$="THE CRE) 1S GETTING WEAKER !!#" :GOSUBS166
WF=16:GOSUB?7 3468
* GOTOS2SH
REC PN(T2#CNo #¢ JL-WkK+ 12
IFRPACT? <ATHENS 268
GOTOS276
FRINT " RUNNING
GOSUBR? 246
St="00 YOU WANT TO PUT THE CREW ON®" :GOSLE?1é
PRINT" HALF RATIOWMS OF "PST?
INPUTIS: 1S=LEFTS¢(1%,1>?
IFI $<" "AND E¢ 2 "MN" THENS226
IFI¢="H" THENS276
HROTI=.5:Ht="¥"
WF=S:GOUSUB?3H8
S#="THE CRE 15
M=PHCT) #HRtT 2 #CN
IFX?PACTOTHEMH=PAtT 9
PACT I=PAtT )—x
IFPACTI=8THENPALT I S-P FP
PRINT RtUS¢13:'S OF sPe los!
PRINT :GOSUBS 208 :MEXT
PRINT :S$=K : GO0SUB71 64
GETI#:1Flé=""THEMS293
RETURN
SHORT OF “iPS 7D
SETTING WEAKER! *" :GOSUB?1 a6
ISSUED"
oO
ny
ss
<é
Deer
pacRy
IAN McKINNELL
CHANNELS
OF THOUGHT
sara
We continue our series on the Spectrum’s
operating system with a look at how the
channels through which the micro sends
2 data to the screen and ZX printer are
4 handling different aspects of the displa
selected and controlled. Our discussion
takes in the use of system variables in
i
Bota
On the Sinclair Spectrum the usual means of input
is the keyboard and the usual outputs are either a
television screen or a ZX printer. In the Spectrum
system, each of these hardware items is called an
input or output channel. The screen, for example,
is referred to as an output channel. The actual data
flowing to or from the computer, in the form of
characters to the screen and information from the
keyboard, is called a stream. A data stream is
capable of being directed into different hardware
channels, assuming that the channel hardware is
capable of handling the stream in the correct
fashion.
We'll be taking a closer look at streams and
channels in our next instalment. In the meantime,
we'll begin our examination of the Spectrum I/O
system with a look at the ‘output a character’
routine, which can be found at address &0010. ‘The
character sent to this routine is forwarded to the
screen or printer, depending upon which channel
has been previously selected. Our diagram shows
CHANNEL K Spectrum
the various channels on an unexpanded Spectrum
and the stream numbers that are associated with
them. Channels are named with a single letter, and
streams are described by a number. In the
unexpanded Spectrum only streams 0 to 3 are
active. The Spectrum OS allocates streams to
channels as shown in the table in the margin.
Thus, when we want to output a character to a
particular device we must first tell the Spectrum
OS which stream we want it to be in. To write to the
screen, we would select stream 2, as this is the one
associated with channel S, and to do this we use a
ROM routine at address &1601 to tell the OS which.
stream we want to select. The stream number is
placed in the A register, before calling the routine,
which then opens up the hardware channel
currently associated with that stream number. For
example, to open channel S for output, we execute
the following instructions:
LD A,2
CALL &1601
Once a channel is open, sending a character to it is
simply a matter of putting the character’s code into
the A register and then executing an RST
instruction to call the routine at address &0010.
This is similar in some respects to the OSWRCH
call on the BBC Micro, with the exception that we
call an address in the ROM directly rather than via
a vector. Channel S operates over the area of the
Channel Crossings
S — Upper part of display
K — Used for output to lower
part of display (used by
BASIC for input and error
messages) and input
from keyboard
P — Printer
Using the Spectrum, data can be transferred to and from
peripherals in ‘streams’ directed via (or ‘associated with’)
different channels. On power-up, channels K, S and P are
associated with streams (0 and 1), 2 and 3 respectively. Thus,
PRINT #2 (i.e. to stream 2 — the screen) is the equivalent of the
normal PRINT command. For example, PRINT #2; “This
message will be printed on the screen”
CHANNELP ZX Printer
KEVIN JONES
THE HOME COMPUTER ADVANCED COURSE 1317
screen that is accessible via the normal Basic PRINT
statements. However, it is also possible to access
the lower two lines of the screen that are normally
used by the BAsic interpreter for the output of error
messages and prompts. As can be seen from the
diagram, these lines form part of channel K.
To send a character to these lines, we simply
open channel K using:
Ber wee
CALL &1601
Similarly, the printer channel can be opened by
loading the A register with 3.
As well as standard characters, we can send
control characters to any channel. Clearly, the
effect that these control characters have will
depend upon the channel in use, but it does mean
that we can do the machine code equivalents of
PRINT AT, PRINT INK, PRINT PAPER and so on. The
following table shows some useful control codes
and their functions when passed to channel S or K.
Obviously, some will have no effect if sent to the
ZX Printer (via channel P).
The extra bytes requested by some control codes
are the parameters that would normally follow
them in a BASIC statement. For example, to execute
_aPAPER 3 command, wed simply send the bytes 17
and 3 to channel S. The following segment of a
program shows the machine code equivalent of
PRINT AT 10,10;“A”.
SE62 18 ld a,2
CDe@116 28 call #1661
SE146 38 ld a,22
D7 46 rst #18
3E6A 36 ld a, 18
D? 56 rst #16
3E6A 78 ld a, 1@
D7 86 rst #18
3E4 1 96 ld a,65
D7 168 rst #16
Cc? 118 ret
Not too difficult, is it? One notable exception in
the list of control codes is the absence of a code for
CLS. To clear the screen, we must call another
ROM routine at address &06DB. It is essential that
channel S is open before we call it, and it’s also
necessary to re-open channel S after we've used it
1318 THE HOME COMPUTER ADVANCED COURSE
SEB2
CDe116
CDDBG 4
3E82
CDO116
C9
;open channel S by
;selecting stream 2
stoken for AT
soutput a char routine
sy co-ordinate
3x co-ordinate
;ASCII] code for “A’
if we want to print anything else to the screen. ‘This
routine will clear the screen:
Id a,2
call #1661 ;open channel §
call #@édb ;clear the screen
ld a,2
call #1661 sre-open channel §S
ret
A useful attribute of the ‘output a character’
routine on a standard Spectrum is that numbers
passed to it that represent Basic keywords are
expanded by the routine and printed in full. Thus:
LD A,249
RST &10
will — assuming channel S, P or K has been
selected — print the keyword RANDOMISE to the
screen.
Graphic commands, such as PAPER, INK and
BRIGHT, issued via the ‘output a character’ routine
are operative only over that sequence of output
characters — they are said to be temporary colour
items. The PAPER n command, if issued outside of a
PRINT statement, is a permanent command, and it
remains operative until another PAPER command
is issued.
The two system variables of interest are called
ATTR-P and MASK-P. The diagram above shows
how these system variables control different
aspects of the display. In ATTR-P, the three bits that
control PAPER and INK colour on a permanent basis
are given the values shown at the top of the next
column. If the bit for FLASH or BRIGHT is set to one,
then that feature is operational. ATTR-P is at
address 23693.
MASK-P is at address 23694 and any bit set to one
in this byte will ensure that attributes on the screen
at the relevant print position are not altered by the
contents of ATTR-P. A further useful system
variable is called PFLAG, and this is found at
address 23697. This variable is also shown in the
diagram. I
INK 9, INVERSE and OVER.
There are two other system variables — ATTR-T
and MASK-T — at addresses 23695 and 23696,
respectively. These are arranged in a similar way to
ATTR-P and MASK-P, but control the temporary
colours (those in use in PRINT statements or those
set up by sending control codes through the RTS
&10 routine).
Using ATTR-P and MASK-P to set up colours is
quite easy; we simply manipulate the contents of
the system variables, altering only those bits that
we need to. This is easily done using the Z80’s AND
and OR logical instructions. So, to execute a
permanent PAPER 1:INK 3 command, we would set
ATTR-P up to: |
PAPER 1 INK 3
using this piece of code:
LD Aj
LD (23693) ,A
RET
Of course, as with BAsIc permanent PAPER and INK
commands, the new colours will not affect what’s
been printed before and the screen will not go to
the new PAPER colour until a CLS command, or its
equivalent, has been executed.
The graphics routines on the Spectrum are part
of the BASIc interpreter program. We will now take
a look at the PLOT and DRAW operations. When
using these routines it is easy to bring about colour
changes by altering the ATTR-P and MASK-P
variables as we have already described here.
The routines themselves are easy to use. The
first of these, PLOT, is called at address &22E5. Its
co-ordinates are passed to it in the BC register pair,
(B holding the y co-ordinate and C the x co-
ordinate). Thus, to execute the PLOT 100,100
command from a machine code program wed
simply execute these instructions:
LD B00 —_;y co-ordinate
LD C00 _;x co-ordinate
CALL &22E5 ;doit
RET
Colour changes are easily performed; the
following routine plots a red dot on the screen, and
then restores ATTR-P to its previous state before
returning to BASIC.
3A845C
FEFF
28F9
3AB85C
ce
Id a, (23693) 3get ATTR-P in A reg
push af sand save on stack
and 248 sclear low 3 bits to 8
or 2 ;set bit 1 to give red ink
id b, 166 sy co-ordinate
Id c,166 3x co-ordinate
call #22e5 3cal) PLOT
pop af srestore original
Id (23693) ,a sATTR-P contents
ret
1@ ; GET-
28 Key:
36
46
38
68
So far we’ve concentrated on channel S. What
about channel K, which also has output facilities?
We can write to the lower part of the screen if we
want to, but what we put there will be overwritten
when the operating system or interpreter
generates a message. For input using this channel,
we don’t even need to call a ROM routine! The
keyboard is scanned once every 20 microseconds
and some system variables are affected,
depending on whether a key is being pressed or
not. Two of particular use are LAST-K (at address
23560), which holds the character code of the last
key pressed, and a system variable at 23556, which
holds the value 255 if a Key is notbeing pressed at
that point in time. The routine here can thus be
used to wait until a key is pressed and then return
the character code in the A register. It simply
checks the contents of address 23556 until it~
doesn’t equal 255. The value in LAST-K will then be
that of the key being pressed. |
CHAR routine
ld a, (23556) ;check to see if
cp 255 ;Key is being pressed |
ir z,kKey sKeep checking
Id a,2356@) sqet LAST-K in Aa
ret
Before leaving the keyboard, let’s look at a
couple of system variables that can be useful.
The other channel on an unexpanded Spectrum
that is of use to us is channel P, which is used by the
ZX Printer. This usually has stream 3 associated
with it. Thus:
LD A,3
CALL = &1601
will open channel P and subsequent characters will
be printed on the ZX Printer if connected. It
should be clear that BAsic makes wide use of the
various channels: PRINT and LIST use channel S,
LPRINT and LLIST use channel P and INPUT uses
channel K. We can also add our own channels to
the three we've discussed here, so that we can
easily access additional devices such as
Microdrives, different printers and other items of
hardware. We'll look at this in more detail in the
next instalment.
THE HOME COMPUTER ADVANCED COURSE 1319
Players In The Drama
We show here three of the
characters that the player
controls in Shadowfire in
different situations. On the left,
Torik is in movement mode with
the possible directions he can
You have just 100 minutes to rescue an
ambassador who, with vital plans, is being
held captive aboard a starship. At your
disposal is a team of six individuals with
various skills... A familiar scenario,
perhaps, but cursor control by means of icon
selection adds an exciting new dimension to
Beyond Software’s Shadowfire.
GHANA
Even the most casual observer will have noticed
that computer games are becoming more
sophisticated. Instead of slavishly reproducing
arcade-style games or being content with text-only
adventure games borrowed from the dungeons
and dragons format, designers of home computer
games are developing a style of their own. These
games combine many of the features of the arcade
and strategy formats to produce an entertainment
which lasts considerably longer than the five
minutes or so of the arcade action and is more
demanding than even the brain-bending puzzles
provided by the adventure.
The scenario of Shadowfire is that the renegade
General Zoff has captured Ambassador Kryxix,
who has the plans for a new type of spaceship
called Shadowfire. You, the player, are charged
with rescuing the ambassador before he is forced
into revealing the plans, and you have 100 minutes
to accomplish your task. To assist you in the
rescue, you have at your disposal six characters,
each with different strengths and weaknesses.
To get aboard Zoff’s spaceship, your team must
be ‘teleported’ down. The only member who can
organise this is the drone Manto, so he must be
sent down first to lay the teleport beam for the
others to follow. However, before you dispatch
them, it is advisable to arm each of the characters
according to their strengths and weaknesses from
the weapons available.
Shadowfire uses a unique system of moving
characters, allowing them to pick up objects and to
fight. Unlike other adventure-type programs that
require the user to type in commands such as “Go
North’ or ‘Pick up laser’, this game enables all
functions to be performed by means of icons and a
moving cursor — somewhat like the operating
system used on the Apple Macintosh. To equip the
1320 THE HOME COMPUTER ADVANCED COURSE
team leader, Zark, with hand grenades, for
example, you first select the Zark icon. Once this is
chosen, the screen changes to a pictorial display of
his strength, stamina and other attributes. On the
right-hand side of the screen are three ‘monitor’
icons representing movement, battle mode and
the objects screen.
By selecting the objects screen with the cursor
(which can be moved from the keyboard, joystick
or light-pen), the screen changes again to display
the objects that are in the same vicinity as the
character as well as a number of ‘activity’ icons. By
choosing the ‘pick up’ icon and then moving the
cursor to the grenades icon, Zark will be equipped
with grenades.
Once armed and teleported down to the
spaceship, the team can begin the search for
General Zoff and Ambassador Kryxix. The
spaceship consists of a number of rooms and
corridors, some of which contain weapons or keys
to enable you to open locked doors, while others
conceal enemy guards who must be destroyed
before you can proceed. If you lack the
appropriate key you can call upon the services of
Sevrina, who can pick locks.
To make the best possible use of the 100
minutes available you should develop a strategy.
Some characters are better in some situations than
others and so it is advisable to have the right
person in the right place at the right time.
What makes Shadowfire such an interesting
game is the cursor control. The player is able to
react to situations with the icon selection process
much faster than he would if each command had
to be typed in separately. Nevertheless, merely
having fast reflexes will not allow you to complete
the mission. Strategy and the ability to make quick
decisions is also vital to a successful conclusion.
He
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iy
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